By Following this guide, you will lose your warranty so keep that in mind before you begin. (Although you have probably already lost your warranty by unlocking the bootloader)

This guide will help you to flash any GSI (Project Treble) ROM onto your Redmi Note 10S, and since TWRP, or any custom recovery for that matter, is not yet available, we have to manually install the GSI roms using fastboot (and fastbootd because of the dynamic partition). The process is lengthy but not that difficult.

It should go without saying that you should backup everything and be prepared to lose your data and potentially soft-brick your phone and the experience with GSI roms is not always known to be stable (although I have not seen any such problems yet aside from a few easy-to-fix bugs) so do all of this at your own risk.

Important to note is that if at anytime you mess stuff up or delete partitions you weren't supposed to (whatever the guides may say, don't erase the product partition, it won't help, you're gonna end up in a bootloop) and soft-brick your phone, as long as you can access fastboot you can easily fix the problem by using the MiFlash tool which will help you get back to square one (you won't have to wait to unlock the bootloader again so don't worry about that)

Now, for the disclaimer:
DISCLAIMER: I'm an amateur in terms of tinkering with android stuff and phones in general. The process mentioned below has worked on my phone but I would still recommend waiting for someone more....experienced to weigh in his/her opinions. If you end up bricking your phone or if things go south, I'm not responsible, that's on you.

NOTE: When I mention fastboot I'm referring to the standard "normal" fastboot and when I mention fastbootd, I'm referring to the dynamic/userspace fastboot which is different (which wiil be indicated by the fastboot screens during either of the modes)

I should mention that using this method I have been able to flash DotOS, Android 12 Beta 5, CorvusOS (my current daily driver) and in my daily driver (Corvus OS), It's passing SafetyNet and the WideVine certification is still at L1 even after rooting, however I have tested neither Google Pay nor Netflix, because I don't use either of them so your mileage may vary, however I did find Netflix in the play store so that is a bit encouraging ig.

Alright, with the appetizers out of the way, let's crack on to the main course:

PART 0: Requirements:-

Please ensure that the following requirements are met:

• A Redmi Note 10S with an unlocked bootloader
• OEM Fastboot driver and Fastbootd driver is installed
• A preferred GSI rom (the file extension should be .img)
• Being prepared to lose your data
• Obtain android platform-tools along with knowledge about how to use it
• Acceptance that the newly flashed GSI rom may/may not be completely stable.
• Fastboot rom from here

If you have all that, you can move on to the next step

PART 1: Get Straight to Flashin':

This guide is rather straightforward because there really isn't much to it.

To begin with, extract the vbmeta.img from the Fastboot rom. Once you have done that, reboot your phone to fastboot mode.
Once the phone is in fastboot mode, on your computer, run the following command:to disable what is the android equivalent of window's secure boot

Once that is done, issue the following command:(No, there is no typo here, this is what needs to be done to enter the fastbootd mode, which is where the real fun begins)

Ensure that you are in fastbootd mode. The blue coloured "fastboot" text at the bottom will be replaced by a yellow coloured "fastbootd"

Okay, now for the most important command, type: Done!

After this, don't forget to factory reset your device by: Note: If you forget to do the factory reset, you're gonna end up with a frankenstein combination of your new and some configs of your previous rom, in this case reboot to bootloader (using power and volume down) and issue the factory reset command (fastboot -w)

And then for the toppings: That's.....about it. You should boot into your chosen rom. For the first time, the boot animation may stick around for a long time (5 minutes or more). However, if, even after an absurd amount of time (15-20), the rom doesn't boot up, try using MiFlash tool to get back to square one and try the whole process again.

Once it boots up, continue with the setup process and you will (hopefully) be at the home screen of your new rom.

PART 2: Le Bugs

Of course, no software is perfect and considering the especially ambitious nature of Project Treble, it is no surprise that you will encounter some bugs. 2 bugs in particular are the disabled headphone jack and the wonky brightness slider.

Here's how I fixed them in CorvusOS (most probably applies in other ROMs as well)

1. Disabled Headphone Jack:

Once you boot up the rom, you'll notice that the headphone jack will not work; the audio will only play on the speaker. To fix this one, you unfortunately need to root your phone (although that's not necessary, try looking into the Device Specific customisations of your rom).
Through your computer, issue the following commands (one-by-one, don't copy-paste them): Your phone will now reboot and once it does, you will see that the headphone jack is functioning normally again.

2. Brightness Slider Fix:

This one does not require any root. However, this does require that you can access the device specific customisations,

So, in my case, I had to go into {Device Name} Settings > Misc features and then tick the Force alternative backlight scale . Once you have done that, reboot and your brightness slider will be working properly again. Also, if you noticed sudden fluctuations after booting or when device goes to sleep earlier, that should all be fixed as well.


Conclusion:

At this point, you should be ready to rock the world along with your phone. Depending upon the rom you have chosen (DotOS for example), your widevine certification and safetynet may be compromised, even after trying out several fixes (which is what happened in my case). So if that happens to you, I suggest that you either change your rom, hunt around for some solutions or just, you know, live with it.
In my case, (As I mentioned before) I'm using CorvusOS where the widevine rating and safetynet are unaffected, with the latter requiring some special fixes.

Hope that helped. I look forward to hearing from you people about any recommendations or discussion. If you have any problem with the process or think that something may be wrong, post it in the replies and I'll be happy to get back to you (depending upon the time, of course)

Pre-flash info:​

• Device name: Xiaomi Redmi Note 10S (secret)
• Earlier rom: MIUI 12.5.15 (Global)
• Rooted?: Yes

Tested image:​

• Pixel Experience 12 Unofficial (phh v401)

Hardware Support post flashing​

Personal Notes:​

• Native screenrecord causes device to freeze and crashes SystemUI. Third party screen recorders work fine tho so it's not a big problem.
• Mobile data extremely rarely works.
• System UI is very laggy, especially the app drawer [thanks @Redmi note 10s user]
• Gcam camera quality looks better than stock, but photo view is very laggy (looks like 15fps) despite video is smooth. Also video shooting only works on main cam the other two are just black.
• There is a noticeable performance drop in games and compile speed in AIDE.
• ============================
• There are many other bugs, most of them are fixable with little tweaks, but these are the most important for me and I didn't find a workaround yet.

After using this rom for about two weeks, I think I'm going back to stock (or LiteE). The features I miss the most are floating apps (there are some alternatives but they are not as practical as stock rom), and especially MOBILE DATA is important. Please tell me if mobile data has been fixed and I will consider coming back to this rom.